The interleukin-2: Interleukin-2 receptor interaction has been one of the most intensely investigated ligand receptor system of the hematopoietic/lymphokine family. Recent findings demonstrate that is system remains an enigma in many respects. Not only is the biochemical mechanism of signal transduction unknown, but within the last year the longstanding 3 dimensional structure for IL-2 has been shown to be incorrect and a third cell surface receptor subunit has been identified. The presence of three cell surface subunits (p55, p64 and p75) makes this receptor system unique among the expanding family of hematopoietic receptors. The knowledge of how each subunit functions with respect to ligand capture, signal transmission and in internalization is essential for the development of ligand based IL-2 agonists and antagonists. We and others have demonstrated that these subunits must function as heterodimers on the cell surface. Only one of the subunits (p55) is capable of interacting with ligand in solution in a manner that resembles cell surface binding. The long term goal of this project is to engineer soluble IL-2 receptor ectodomains that associate in a directed multimeric fashion in solution by exploiting the wealth of knowledge concerning the specificity and stability of coiled - coil (Leu Zipper) molecular recognition. The Specific Aims of this project are: A. To engineer and express fusion proteins that combine each receptor ectodomain with coiled- coil recognition sequences designed to specifically direct subunit interaction in solution. B. To characterize receptor subunit complexation using biophysical methods. C. To quantitate the ligand binding properties of the soluble receptor complexes and compare them to the equivalent cell surface structures. D. To optimize the designs of the soluble complexes such that bind ligand will greatly facilitate the characterization of this ligand receptor system. In addition, these results will demonstrate the feasibility of this approach for the study of other members of the hematopoietic receptor family.